Contact lenses are commonplace today. Most individuals with average refractive errors can quickly and easily acquire and use these lenses in place of prescription eye glasses. This is not true, however, for individuals who are presbyopic (i.e., those requiring multi-focal visual correction) or for those individuals with structural eye abnormalities. These individuals are left with little choice in selecting comfortable, effective contact lenses. Lenses which are available typically encumber these wearers with other difficulties, and are usually very expensive. Presbyopic individuals, for example, who choose to wear soft contact lenses are usually fitted in a "monovision" mode, where one eye is corrected for near vision, and the other eye is corrected for far vision. Notwithstanding the availability of such multifocal lenses, such lenses are not versatile in supplying simultaneous clear distance/near vision and are not commercially successful.
The long felt need to develop more versatile multifocal lenses has led designers to "pinhole" contact lenses (e.g., PCT Publication No. W095/08135 published Mar. 23, 1995). These lenses endeavor to utilize the known theories of pinhole imaging as a method to reduce or eliminate visual deficiencies. An annular mask with a clear center aperture of various sizes (conventionally up to 4 millimeters in diameter) is used to increase the depth of focus of presbyopic individuals. Unfortunately, the utility of this technology has been diluted because of designs and approaches inappropriate for effective corrective refractive prescriptions. For upwards of 50 years, pinhole contact lenses have been under consideration, yet they remain today commercially unsuccessful and largely unavailable. One of the principle reasons conventional pinhole contact lenses have been ineffective is that they suffer from diffraction effects at the sharp demarcation where the pinhole aperture stops and the opaque mask surrounding the aperture begins.
"Multiple Focal Contact Lenses", as described in U.S. Pat. No. 3,794,414, was one attempt to develop small-aperture contact lenses. This approach combined a pinhole-like aperture with radial slits and scalloped masking regions on a contact lens supposedly to correct both peripheral vision and the effects related to decentered contact lenses. The masked contact lenses were made from a rigid substrate, and "floated" on the eye, creating a need for apertures over a large portion of the lens. The disclosed designs though (i.e., the use of scalloped patterns and radial slits) actually encourage diffraction effects at the retina. This reduces image quality. The purpose of pinhole aperture correction is to correct geometrical aberrations in excess of diffraction. Therefore, the benefits achieved according to that patent by incorporating the pinhole aperture, are likely to be offset by undesirable diffraction effects.
Pinhole correction together with the normal functioning of the human pupil is considered in U.S. Pat. No. 4,955,904, which presents a masked intraocular lens surgically implanted within the eye. The patent, entitled "Masked Intraocular Lens and Method for Treating a Wearer With Cataracts", affords cataract wearers some form of vision correction through surgery. The intraocular lens is masked to form a pinhole that accommodates the function of the human pupil under different lighting conditions. However, the intraocular lens of that patent is also likely to be offset by undesirable diffraction effects created by the sharp demarcation at the junction of the pinhole and the opaque mask. U.S. Pat. No. 5,245,367 issued to Miller et al. discloses an annular mask contact lens wherein the optical opacity of the mask region can vary within the lens.